Radio frequency modulated optical fibres systems, such as those described in U.S. Pat. No. 5,917,636 enable a mobile network to be built covering wide areas. These systems are capable of accepting continuous increase in demand in terms of data rate and free spectrum range and of expansion of the equipment and infrastructure installed. The increase of frequency of these systems enables a reduction in the size of the antennae and devices used in the radio frequency part of the network and an increase in the bandwidth of the whole system. For data rates of 155 MBit/s and higher, two spectral ranges of 62-63 GHz and 65-66 GHz, which are still unallocated by ITU, have been identified within the framework of the NBS project (RACE 2067) as particularly suitable for such systems.
The present invention is applicable to these frequency ranges and other frequencies, especially but not exclusively in the so-called millimetric radio frequency ranges, where the wavelength of the radio signal in free air is of the order of one to ten millimeters. Such radio signals may be subject to strong absorption by the atmosphere and by common building materials, so that it is of particular interest transmission for the radio signals to be modulated onto an optical signal that is guided in an optical fibre from a source- to a so-called pico-cell, where the radio signal may be detected and, if desired, radiated over a relatively short distance.
In order to minimise problems resulting from chromatic dispersion in the optical fibres, generation of the modulated optical signals is preferably obtained by combining two phase-correlated optical carriers which are generated in a dual-frequency laser transmitter with a frequency offset equal to the desired microwave frequency of the radio carrier signal. The two optical signals are then transmitted through optical fibre and the radio signal is recovered in a detector. Systems of this kind are described for example in the article “Chromatic dispersion in fiber-optic microwave and millimeter wave links” in IEEE Transactions on Microwave Theory and Techniques, volume 44 number 10 pages 1716 to 1724 and the article “Multifunctional fiber-optic microwave links based on remote heterodyne detection” in IEEE Transactions on Microwave Theory and Techniques, volume 46 pages 458 to 468.
An example of a communication system of this kind, which may be used in a system in accordance with the present invention, is shown in FIG. 1 and comprises, at the transmitter end, a master laser 1, connected by optical fibre 2 to a radio frequency modulator 3, which in turn is connected by optical fibre 4 to a beam splitter 5. The two optical signals from the beam splitter 5 are connected by optical fibres 6 to two slave lasers 7 tuned to frequencies spaced apart by an amount equal to the radio signal frequency, which act as filters to generate optical signals with a high degree of phase correlation and spectral purity. The outputs of the slave lasers 7 are connected to a recombination device 8 before transmission over the optical fibre output line 9. At the receiver end, the system comprises a detector 10 that receives the optical signals in the optical fibre line 9 and demodulates the radio signals.
It is important for the modulator 3 and detector 10 to provide efficient generation and detection of the radio frequency modulated optical signals. Many proposals have been made based on the use of semiconductor materials in which the optical and radio waves interact. Thus, U.S. Pat. No. 5,917,636 discloses the use of an electro-absorption layer comprising a multi-quantum well structure of InGaAsP, whose physical dimensions are very much smaller than the wavelength of the radio signals used.
The article “Ultrawide-band/high frequency photodetectors” in IEEE Transactions on Microwave Theory and Technique, Volume 47 pages 1265 to 1281 discloses the use of photodetector material for the interaction in conjunction with various wave-guide and travelling-wave structures for the radio frequency signal. Once again, the dimensions of the detector material are very much smaller than the radio frequency wavelength.
The article “High frequency polymer modulators with integrated finline transitions and low Vπ” in IEEE Photonics Technology Letters, Volume 11 pages 54 to 56 discloses the use of a non-linear optical polymer material in a radio frequency modulator for the optical signal, the material being disposed between two electrodes terminating a radio frequency microstrip wave-guide. The radio signal detector described in that article is a photodetector.
In the known systems, problems arise with the efficiency of the systems and the requirement to provide a stabilised voltage for photodiode and phototransistor devices, whose efficiency reduces further at higher frequencies, especially above 100 GHz, for example.
We have found that critical factors in the design of suitable radio frequency modulator and detector devices for use in such systems, especially passive devices, relate to the choice of electro absorption materials and also to the coupling between the various parts of the devices.